Choosing a baseline period for NSR project emissions accounting

What the baseline does

For a project at an existing major source, Step 1 of the major modification test compares each unit's emissions after the project with its baseline actual emissions. A higher baseline means a smaller calculated increase, which is why the rule's choices about the baseline period matter so much.

The federal PSD rule defines baseline actual emissions at 40 CFR 52.21(b)(48). Most states use the same structure in their SIP-approved rules, but read the version that applies to your project.

The window and the lookback

The baseline is an average annual rate over a window of consecutive months that the owner chooses, inside a lookback before the project. The federal rule describes it as an average rate “during any consecutive 24-month period selected by the owner or operator”, and the lookback differs for electric utility steam generating units and other existing units (40 CFR 52.21(b)(48)(i) and (ii)).

For units other than electric utility steam generating units, the lookback runs back from a defined date: the rule refers to the period “immediately preceding either the date the owner or operator begins actual construction” or the date a complete application is received, whichever is earlier. Settle that date first; it fixes which months are eligible.

One window for all units, one per pollutant

When a project changes several units, the rule says “only one consecutive 24-month period must be used to determine the baseline actual emissions for” the units being changed. You can't pick each unit's best period separately. You can, however, use a different period for each pollutant: “A different consecutive 24-month period can be used” for each regulated NSR pollutant.

So the real question is which single window gives the highest combined baseline for the units in the project, pollutant by pollutant. Scanning every eligible window answers it; the NSR Baseline Calculator does that scan.

Adjust the numbers before averaging

Read the adjustment provisions in (b)(48) for your unit type. For other existing units, the federal rule:

• Includes fugitive emissions to the extent quantifiable, and emissions from startups, shutdowns, and malfunctions
• Excludes emissions that were not in compliance with legally enforceable limits during the period
• Adjusts downward for emissions that would exceed limits the unit must currently meet, such as a newer standard
• Does not allow a period “for which there is inadequate information” for determining annual emissions

Projected actual emissions and the exclusion

On the other side of the comparison, projected actual emissions are the highest annual rate the unit is projected to emit in the period after the project that the rule specifies, based on all relevant information. The owner may exclude the part of “the unit's emissions following the project that an existing unit could have accommodated” during the baseline period and that is unrelated to the project, such as growth in demand (40 CFR 52.21(b)(41)(ii)(c)). Electing potential to emit instead removes that exclusion.

Write down the basis for both numbers: data sources, business projections, and why any excluded emissions are unrelated to the project. That record is what an agency reviews.

Keep the record after the decision

Projects that rely on projected actual emissions can carry recordkeeping and, in some cases, reporting obligations for years after the change (see 40 CFR 52.21(r) and your state's rule). Put them on the compliance calendar the day the project is approved.